1. Field of the Invention
This invention relates to magnetic recording mediums of the type which comprise a magnetic recording layer on a non-magnetic substrate.
2. Description of the Prior Art
The magnetic recording medium of the type mentioned above is ordinarily made by applying a magnetic paint comprising magnetic powder and a binder, to a non-magnetic support and drying the applied paint.
In recent years, magnetic recording mediums having high magnetic density and high durability are greatly demanded. However, conventional magnetic recording mediums of the type in which a magnetic recording layer is merely formed on a non-magnetic support cannot satisfy the recent demand. Accordingly, there have been proposed magnetic recording mediums which comprise an intermediate or undercoat layer between the magnetic recording layer and the support. The intermediate layer is made, for example, of phenolic resins, epoxy resins, polyester resins, or mixtures thereof.
However, these intermediate layers made of resins alone are not satisfactory in view of the surface characteristics of the magnetic recording layer and the adhesion to the support. With regard to the adhesion, it was found that the undercoat layer had to have an appropriate degree of solubility in a magnetic paint used to form the magnetic layer. For instance, when an undercoat layer is readily soluble in a magnetic paint and has a thickness as small as about 0.1 .mu.m or below, the layer is dissolved by means of a magnetic paint and disappears. On the contrary, when the undercoat layer has a thickness over 1 .mu.m, it is partially dissolved in a magnetic paint, thus causing the resulting magnetic layer to be irregular on the surface thereof. In addition, as the undercoat layer increases in thickness, the thickness of the magnetic layer or non-magnetic support should be decreased. This will produce a number of problems in practical applications.
If the undercoat layer was completely insoluble in a magnetic paint, it could not be expected to improve adhesion of the magnetic layer. Thus, it becomes meaningless to provide the undercoat layer.
When an undercoat layer made only of a resin such as a phenolic resin is formed between the magnetic layer and the non-magnetic support, chargeability on the surface of the magnetic layer increases because the surface electric resistance of the undercoat layer is as high as 1.times.10.sup.11 ohms/square, thus lowering the travelling performance of the resulting medium.
In order to overcome the above disadvantage, U.S. Pat. No. 4,275,113 describes a magnetic recording medium which comprises an intermediate layer of carbon black dispersed in a resin binder. However, this medium requires large amounts of carbon black in order to lower the surface electric resistance sufficiently, and peeling strength of the intermediate layer is relatively low, which presents a problem on durability of the medium.